Definition

Connectivity of the cards, i.e. whether they are close to one another in value, and hence able to build straights.

Suitedness, or whether there are many cards of the same suit enabling flushes.

Value of the cards, i.e. how many cards are high cards like aces or kings, since these are played more often and so opponents are more likely to have hit them. In hi/lo variants, it might also be important whether there are many low cards that enable lo hands.

In general, one can say that the "goodness" of a flop depends on how many draws and what kinds of finished hands are possible with it. Players tend to play with cards that are suited, connected, or high, so it is important to know which of these is possible given the flop.